Project Summary Each year, poorly coordinated care in the US healthcare system results in an estimated $25-45 billion in unnecessary healthcare spending due to avoidable complications and readmissions, and over $10 billion in duplicative testing. Care coordination between organizations, or ?inter-organizational care coordination,? is particularly challenging since there are more opportunities for communication breakdowns when information travels across organizational boundaries. One of the most recent healthcare reforms targeting inter- organizational care coordination is the Accountable Care Organization (ACO) model. The ACO model unites disparate healthcare organizations under a single ?virtual? organization that is responsible for the care of a designated population. Medicare started two ACO programs in 2012, in the hope that participating organizations will be motivated to improve inter-organizational care coordination. Although the impact of ACOs on patient outcomes and costs have been studied, little is known about the impact of ACOs on inter- organizational care coordination, and whether changes in inter-organizational care coordination are resulting in desired cost reductions and quality improvements. This retrospective longitudinal study investigates the impact of hospital participation in ACOs on inter- organizational care coordination and cost and quality outcomes for patient transitions from hospital to post- acute care (PAC). Using a national dataset on general acute-care hospitals in the US from 2012-2016, this project addresses 2 aims. Aim 1 is to evaluate the effect of inter-organizational care coordination on the relationship between hospital ACO participation and cost and quality outcomes (per patient Medicare spending, all-cause readmissions, and ambulatory sensitive readmissions). Aim 2 examines whether electronic health information exchange (HIE) engagement and patient sharing patterns between hospitals and PAC providers contribute to improved care coordination within ACOs. Findings from this study will 1) reveal whether ACO participation improves care coordination for hospital-PAC transitions, and whether those improvements contribute to lower costs and better patient outcomes, 2) assess whether hospital engagement in health information exchange and more concentrated hospital-PAC networks predict a stronger relationship between ACO participation and improved inter-organizational care coordination during PAC transitions, and 3) inform policy efforts aimed at improving ACO design and other health reforms seeking to improve patient outcomes and healthcare costs, two AHRQ priority areas.